In many painting systems for automobiles, various household electrical applicances, and the like, the products are coated or painted in enclosed areas referred to generally as paint spray booths. Generally a significant portion of the paint is oversprayed, that is, not transferred to the object being coated. Such waste paint is generally referred to as oversprayed paint.
The term "paint" as used herein includes paint in its ordinary sense, that is, a mixture of pigment and a suitable liquid vehicle that is reasonably fluid and provides a thin and adherent coating when spread on an appropriate surface. The term "paint" as used herein also includes other paint-like fluid coating materials. Hence the term "paint" as used herein refers to paints, lacquers, varnishes, base coats, clear coats and the like.
Paint spray booths are generally comprised of a chamber, a duct system for passing an air stream down through the chamber, and a sump or recirculating water system, the bulk of which is located at the bottom of the chamber. The air stream is a type of wash stream that generally captures oversprayed paint and carries it towards the sump. The water within the sump is generally treated so that the oversprayed paint, when it is carried to such water, is formed into a paint sludge. Such paint sludge typically floats at or near the surface of such water, and can be removed by skimming or similar means.
The tendency of such paint sludge to float also permits reasonably efficient water recycling, because reasonably sludge-free water can be drawn from or near the bottom of the sump. In some installations, water at least partially withdrawn from the sump is pumped upward to a point where it cascades down the sides of the chamber, forming a vertical curtain of water that protects the chamber walls.
It is extremely important to the efficient operation of such paint spray booths, and to the efficient waste treatment of such oversprayed paint wastes, that the oversprayed paint be detackified when it contacts the water system, or waste water system. Such contact may be made with the water present in the booth or with the water recirculated to the water curtain that protects the chamber walls. By detackification is meant herein that the adhesive properties (tackiness) of the paint overspray are minimized or eliminated. It is severely undesirable to have such overspray adhere to the walls of the paint spray booth chamber or the walls of the sump. It is severely undesirable to have such paint overspray adhere to any surface that may be encountered in the spray booth, or during its removal from the spray booth and subsequent disposal.
It is also advantageous if a paint detackifying agent promotes the formation of paint sludge that can be efficiently removed and dewatered. Removal and dewatering of paint sludge is affected by a plurality of sludge characteristics, including besides detackification, cohesiveness to promote easy sludge removal, low water content for total volume reduction, low or no solvent encapsulation to reduce the hazardous properties of the sludge and the like. High solids and minimum solvent are requirements under most sludge landfill disposal regulations. An initial dewatering of the sludge occurs during sludge removal, and the sludge collected may be further dewatered using such devices as a plate and frame filter, a vacuum filter, a centrifuge, a twin belt press or the like. For effective dewatering during the sludge removal, and when the collected sludge is further dewatered using a dewatering device, the sludge paint must be well detackified.
It is also desirable that a paint detackifying agent have a high loading capability in the system in which it is used. A detackifying agent of high loading capability is one that can withstand a high volume of oversprayed paint before exhaustion, that is, the point at which detackification performance falls to an unacceptable level. In a process where the paint detackifying agent is fed to the water system continuously, for instance on a constant feed mode, the loading capacity of the detackifier will determine the maintenance feed rate. A detackifier with a high loading capacity will require a lower maintenance feed rate. If a system is not continuous and is instead run to, or close to, the point of exhaustion of the detackifier present, a detackifier of high loading capability would permit the water system to handle a higher volume of oversprayed paint before shutdown. A paint detackifying agent of high loading capability (amount of oversprayed paint detackified per unit additive) is cost efficient, and in a continuous operation the loading capability of of the paint detackification agent is seen from the lower maintenance levels required.
It is also highly desirable that the detackifying agent be dispersible in the waste water system with reasonable efficiency. An additive that requires auxilliary equipment for mixing and dispersing would increase the operation costs and serving time required. Hence it is desirable that the detackifying agent can be provided in suitable form for charging directly to the waste water system, and be dispersed homogeneously therein rapidly, at normal operating temperatures and without agitation other than that provided by the normal operation pumping action and the like.
As noted above, the paint sludge that is formed generally floats on or near the top of the water present in the sump. Generating such floating sludge, and reducing the amount of sinking sludge, is desirable and extends the time between pit cleanings. Generation of floating sludge in some systems is assisted by air flotation means. The removal of the sludge on or near the top of the water may be accomplished using one or more of a variety of systems, including without limititation by skimming, by screening or straining, by dispersing the sludge in the water of the sytem and then discharging a portion of such water, and the like. Sinking sludge is generally removed at given periods. In some operations the sludge removal is continuous and such continuous sludge removal systems frequently use a pedal wheel, skimmer, or a side-stream sludge removal when an air assisted floating unit is used to separate the sludge from the water. As noted above, efficient sludge removal is dependent in part on sludge characteristics. The paint should be well detackified and the sludge preferably should be cohesive.